1. Field of the Invention
The present invention realtes to a liquid droplet projection apparatus used for an ink jet printer and the like, and in particular to a liquid droplet projection apparatus in which liquid droplets are ejected from a nozzle by changing the volume of electro-mechanical transducer means in accordance with an electrical signal.
2. Description of the Prior Art
In conventional liquid droplet projection apparatuses of this type such as disclosed in U.S. Pat. No. 3,946,398, a liquid such as ink is supplied from a liquid supply tank into a liquid droplet projection head through a liquid conduction tube.
When a piezo-electric element of the electromechanical transducer means is impressed with an electrical signal thereby to excite the piezo-electric element under the condition that the liquid is filled in the nozzle of the liquid droplet projection head, the electro-mechanical transducer means and a common liquid tank, the body with the piezo-electric element bonded thereto is displaced so that the volume of the electro-mechanical transducer means is sharply reduced, thus ejecting liquid droplets from the nozzle by the resultant pressure wave.
The piezo-electric element of the liquid droplet projection head of these conventional types of liquid droplet projection apparatuses is generally supplied with an electrical signal of a fixed amplitude and a fixed pulse width to eject the liquid droplets.
In the case where ejection of the liquid droplets from the nozzle is suspended or interrupted followed by resumption of ejection and also the period of the interruption is longer than twice the period of the electrical signal which is applied to the piezo-electric element, and of which the driving frequency is high, then some droplets ejected from the nozzle after the interruption are larger in radius and other droplets are smaller than normal for unexplained reasons. This change in droplet size by the operating conditions of the devices in spite of a fixed electrical signal applied for generating uniform liquid droplets of a predetermined size, has been found to reduce the quality of characters and patterns recorded by these various sizes of droplets.
In particular, the degradation of the quality of the record due to the reduction in the radius of droplets is so great that if the droplets disappear, the characters may become difficult to be deciphered.